Question:
Case stydy about the Quality control for the process of ground improvement using the jet grouting technique.
Jet grouting can be defined as a process that is being used in the construction industry and this process is used with the help of a high kinetic energy jet of fluid in order to break and loosen the soil so that it can be mixed with thin slurry that is produced (Alp and Rasin, 2004). Jet grouting is known as a grouting method by which soils are grouted and it is performed with the help of a grouting monitor that are attached at the end of the stem which is being used for drilling. With the help of this monitor, the maximum possible depth can also be seen. The jet grouting method can be classified into three different classes and these different types of jet grouting methods are used in different cases and it depends on the types of the soil at which the work is to be performed. The three classes of jet grouting are known as the single fluid system, the double fluid system and the triple fluid system. The single fluid system is also known as slurry grout jet whereas the double fluid system is known as the slurry grout jet that is surrounded by an air jet (Greenwood, 2008). On the other hand, the triple fluid system is known as the system in which water jet is surrounded by an air jet along with a lower amount of grout jet. All the three classes of jet grouting system contain three phases and these are to be performed in proper sequence.
The quality control procedure is known as a set of or a group of procedures that are used in order to ensure the quality of a particular process, service or product is under control or not. In order to maintain the quality or control the quality of a process or product or in other words, in order to implement the quality control system in the industry some standards are to be made.
The construction and installation of the foundation plate base of a new stadium in the Prikubansky interurban district of the city of Krasnodar in Russia is taken into consideration doe the analysis of quality control in jet grouting ground improvement processes.
The terrain of the construction site was quite flat and the ground comprised of hard and stiff loam, silty sand, semisolid clay and medium & gravel sand. Analysis of various geotechnical and seismological conditions of the ground showed two primary problems that might arise due to any seismic or excessive weighting loads. These are vibroliquefaction and vibrocreep of the lower tight layers of silty sand and excessive porous nature of layer of brown clay, in case of any kind of seismic activity. Thus, in order to improve the quality of the ground and its underlying layers, double jet grouting technique was used to achieve stiff characteristics of the foundation of the stadium.
Several quality control mechanisms were implemented to ascertain the initial, changing and final quality of the ground. During the process of foundation stabilization using jumbo or double jet grouting, the best process variables that can result in the formation of the required diameter of bored hole by the jet elements were the main factors that were kept in consideration. Also other important factors like the physical and mechanical properties and characteristics of the resulting soil-cement mixture in the foundation were determined to ascertain of the sturdiness and load bearing capabilities of the foundation. The construction team ensured that the diameter of the foundation elements must not be lower than 1200 millimetre and the compressive strength of the elements after a settling period of 28 days must not be less than 4 megapascals.
Four soil-cement elements were constructed with the double jet grouting process at a depth of 13 metres from the ground surface. The drilling depth was determined by the already obtained engineering and geological data of the construction site ground.
Vital quality control and assessment measures were undertaken during the construction of the foundation elements with about 900 kg of Portland cement for every one running metre of the stable soil. After aging for about seven days, the established foundation elements were drilled to determine their actual diameters and it was found that the actual diameter of all the elements was higher than 1200 millimetre, thereby satisfying the quality requirements.
Some test drilling and foundation laying were also done using lower amounts of cement quantity, typically 600 and 650 kg near the actual construction site, in order to find out whether the set objective can be achieved while cutting down on costs. But the observed diameter was found to be lower than the set target of 1200 millimetre and thus the amount of cement to be used in the foundation laying process was determined as 900 kg.
The samples of soil-cement mix obtained by boring and grouting were also analysed and test samples were taken from various depths from the soil surface after a hardening and settling period of 7 to 14 days. These samples were tested for compressive strength and the stress-strain modulus using the universal testing machine Zwick2-250. After obtaining the test results, statistical analysis of the obtained data were used to define the average values of the compressive strength and the stress strain values of the core samples. The soil-cement compressive strength of all the underlying layers of the ground was found to be within the required parameters and values. (Galina G., Oleg and Ilgiz, 2013)
The need of quality control team is needed in the jet grouting works in order to measure the proper ground settlements and it also can be understood that the settlements are excessive or not. There are different types of jet grouting methods that can be applied in a particular type of soil but the most suitable technique that has been chosen in the stadium construction process so as to maintain the quality of the process and also to meet the requirements properly, is the double jet grouting technique. In the quality control process of the double jet grouting process, after testing all the materials, tools, equipments and machineries, some tests are made and these are made a number of times in order to make sure that the results that are found are correct. All the chemicals and other elements that are used in the jet grouting system also must be checked properly in order to ensure that they are meeting the required quality or not. Then a period comes and this period is known as the adjustment period and in this period all the outcomes of the quality control process that are not meeting the requirements are adjusted in order to meet the objectives and maintain the quality of process. In this phase the control chart is used. A control chart can be defined as a chart in which all the data are graphically represented and by observing which one can easily understand that the process or the activity is under control or out of control. The control limits are to be standardised before observing the activities in the control chart or else it cannot be understood that a process is under control or not. After plotting data into the control chart and understanding it, proper interpretation of the data and analysis were performed in order to improve the overall quality of the foundation laying process. In order to analyse the data that are found from observing the activities related to the jet grouting process, statistical analysis measures were used and through this method the error and % error in all the vital processes were found. This helped to improve the quality of the processes that are not meeting proper qualities. While executing the jet grouting the diameter of the bored hole was properly controlled. Maintenance of the quality and amount of the diameter of columns made by jet grouting methods were done by monitoring certain factors that can have an impact on the diameter of the columns. (Thomas, 2013). The type and strength of soil were taken into consideration and the soil erosion property of the site was checked. The diameter though mainly depended on the method of jet grouting that is being used in the work process, that is, double jet grouting .(Thomas, 2013).
The Jumbo Jet Special Grout method is one type of double jet grouting method where two fluid jets are used along with proper cement grout and air. In this technique, cement grout mixtures are injected under high pressure using a cone of compressed air that covers the grout injection. This air helps to reduce frictional loss and helps the cement to penetrate further, leading to the creation of large diameter columns.
The diameter of the ground hole created by the jet grouting process is usually affected by soil properties and other variables like lifting speed, grout pressure, nozzle diameter etc. Hence, these factors and parameters were critically monitored for quality assessment and control. The procedures used to evaluate & control quality of operations were applied to the samples of grout cubes & spoil cubes; spoil density reading to check for strength compatibility with respect to the sections extracted for trial purposes.
Double fluid jet grouting process has been used in this context to improve soil strength & lower the permeability of the soil’s silty layer of sand. Real time monitoring and quality control were performed during the whole process to prevent any heaves from occurring. Dewatering systems were also installed, before the ground was drilled in order to prevent any excess water seepage into the bored hole, which might lead to weakening of the soil layers. Then, certain tests were performed to assess the quality of the double jet grouting process. Coring and strength testing were done in production jet grout columns and some samples were selected from each cored column for strength tests. Usually, this test is done 21 days after cement grouting and installation of columns. Unconfined compression tests are then performed on the extracted samples in accordance to the guidelines laid down in ASTM C42. (Brajesh, 2016)
Visual inspections were routinely done and the diameters of the soil bores were measured at every 500 mm, where it was observed that diameter of columns laid down in the soil using double jet grouting tends to get narrower near about the ground water table level. Visual quality control tests were followed by integrity testing. Integrity of the columns constructed was tested thoroughly to assess the quality and condition of the soil after grouting. (Giuseppe and Joanna, 2012)The PDI pile integrity tester was used as a technical tool or instrument for this purpose. Quality control is extremely essential to carefully monitor the materials used in the soil pumping & mixing process. The grout mix used to improve the ground was tested for density measurement suing hydrometers and for viscosity using Marsh Cone. The whole installation process was carefully controlled and observed to maintain quality at every step. Vital parameters such as flow-rate, pressure and total volume of concrete injected into the soil were monitored suing automatic instruments. Certain other parameters such as grout rotation speed and withdrawal were also kept under observation during the foundation settling process. One vital point is that for the case of deep soil mixing like the one done in the stadium construction process, coring and unconfined compressive strength testing are mandatory. (James C. and Wen-Chieh, 2014)
These types of comprehensive and all encompassing quality control testing and verification techniques are incorporated in order to proactively identify and resolve any type of potential problems in the ground improvement process.
References
Alp, G. and Rasin, D. (2004). Ground Improvement By Jet Grouting Technique For Foundations Of A Natural Gas Combined Cycle Power Plant In Turkey.
Brajesh, M. (2016). A Study on Ground Improvement Techniques and Its Applications. International Journal of Innovative Research in Science, Engineering and Technology.
Giuseppe, M. and Joanna, B. (2012). Analysis of Foundations Reinforced with Jet Grouting. Journal of Geotechnical and Geoenvironmental Engineering.
Greenwood, D. (2008). Discussion: Theoretical modelling of jet grouting. Géotechnique, 58(6), pp.533-535.
James C., N. and Wen-Chieh, C. (2014). Quality control of double fluid jet grouting below groundwater table: Case history. Soils and Foundations.
Richard, F. (2005). Review of the Jet Grouting Method.
Thomas, K. (2013). The control of column diameter and strength in Jet Grouting processes and the influence of ground conditions’.
Galina G., K., Oleg, M. and Ilgiz, K. (2013). Experience in Application of “JET Grouting” for Installation of Substructures of Estates. Frontier in Geotechnical Engineering (FGE) Volume 2 Issue 2, June 2013.
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